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    SubjectRe: [ANNOUNCE] mdb: Merkey's Linux Kernel Debugger 2.6.27-rc4 released
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    >> Paul E. McKenney wrote:
    >>> On Thu, Aug 21, 2008 at 01:02:48PM +0200, Peter Zijlstra wrote:
    >>>> On Thu, 2008-08-21 at 12:57 +0200, Stefan Richter wrote:
    >>>>> Peter Zijlstra wrote:
    >>>>>> On Wed, 2008-08-20 at 20:50 -0600, jmerkey@wolfmountaingroup.com
    >>>>>> wrote:
    >>>>>>
    >>>>>>> volatiles left in the code due to the previously stated
    >>>>>>> (and still present) severe breakage of the GNU compiler with SMP
    >>>>>>> shared data. most of the barrier() functions are just plain
    >>>>>>> broken
    >>>>>>> and do not result in proper compiler behavior in this tree.
    >>>>>> Can you provide explicit detail?
    >>>>>>
    >>>>>> By using barrier() the compiler should clobber all its memory and
    >>>>>> registers therefore forcing a write/reload of the variable.
    >>>>> I hope Jeff didn't try mere barrier()s only. smp_wmb() and smp_rmb()
    >>>>> are the more relevant barrier variants for mdb, from what I remember
    >>>>> when I last looked at it.
    >>>> Sure, but volatile isn't a replacement for memory barriers.
    >>>
    >>> Let's face it, the C standard does not support concurrency, so we are
    >>> all in a state of sin in any case, forced to rely on combinations of
    >>> gcc-specific non-standard language extensions and assembly language.
    >>>
    >>> Could be worse!!!
    >>
    >> Nevertheless, an analysis of which particular parts of code generation
    >> are insufficient if one particular volatile qualification is removed is
    >> IMO likely to turn up places in mdb where a clearer or/and more
    >> efficient implementation is possible. (Based on what I saw a few
    >> revisions ago; I haven't looked at the current one yet.)
    >> --
    >> Stefan Richter
    >> -=====-==--- =--- =-=-=
    >> http://arcgraph.de/sr/
    >>
    >
    > I used the smp_wmb() functions. I noted a couple of things. a) some of
    > these macros just emit __asm__ __volatile__ into the code so why not just
    > say "volatile" to begin with b) smp_wmb() in some cases worked and in
    > other cases jut optimized away the global reference. c) I can go back and
    > break the code again by inserting them and building broken assembler d) I
    > ave been doing hardware and software design since the early 1980;s, I
    > invented SMP affinity scheduling, and yes, I understand barriers and this
    > concept of instruction score-boarding and optimization very well -- its
    > not an excuse for a busted C compiler.
    >
    > It did not break all the places in the code, but broke enough for SMP to
    > lock up and fail, It turned global variables into local variables. If
    > you want me to reproduce this I can but it will have to wait til this
    > evening
    > because I have some product releases to get out the door at Omega 8 today.
    >
    > It's simple to reproduce. Take away the volatile declaration for the
    > rlock_t structure in mdb-ia32.c (rlock_t debug_lock) in all code
    > references and watch the thing lock up in SMP with multiple processors in
    > the debugger each stuck with their own local copy of debug_lock.


    Even if you use the smb_wmb() macros around the debug_lock, the compiler
    still optimizes the debug_lock into a local variable. After watching the
    broken behavior, the fact that some of these macro's emit __asm__
    __volatile__ anyway, I just left the vars declared volatile. Its a
    debugger, so its probably ok for the kernel debugger to use some volatile
    data.

    Jeff

    >
    > The barrier functions do not appear to work in all cases.
    >
    > Jeff
    >
    >




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